An image forming apparatus of a tandem type is conventionally provided, in which photoconductors are provided individually for respective colors such as black, cyan, magenta and yellow. The photoconductors are arranged along the rotational direction of a paper conveyor belt (i.e., an image carrier), so that images of respective colors held on the photoconductors can be sequentially transferred to paper on the belt.
In this type of image forming apparatus, color registration for aligning images of respective colors is important, because the resultant color image may include a color shift due to color registration errors (i.e., due to displacement of the images of respective colors). In view of this, it has been proposed that color registration errors be detected and corrected for respective colors in the image forming apparatus.
Specifically, a registration pattern (i.e., a pattern used for alignment) including a plurality of marks of respective colors such as black, cyan, magenta and yellow is formed on the belt, so that the marks of respective colors are arranged spaced apart along the rotational direction of the belt.
The positions of marks of each color on the belt, which indicate displacement of an image to be formed of the color, are detected by an optical sensor, and the displacement is corrected based on the detected positions. In this way, displacement of an image to be formed of each color is corrected in the rotational direction of the belt.
However, in some cases, displacement in the direction perpendicular to the traveling direction of the belt and parallel to the belt surface should be also corrected.
Further, the belt in itself involves displacement or movement fluctuation when rotating. That is, its displacement or movement varies depending on the areas of the belt. Therefore, the above correction depends on where the registration pattern is formed on the belt. That is, the correction may fail to be made with accuracy depending on where the registration pattern is formed.